2005 YZ450f 4th gear slipping?

[nohope]

I just purchased this bike mostly to resale but after this issue I may be stuck with it. I installed new ebc heavy duty clutches in it recently and I have only rode down my 450 ft drive way and never get it 4th gear. Today after a short trip down the road it began to slip or pop out of gear in 4th gear. After some research on here I have realized that it has either a bent shift fork or 4th gear is bad or even both.  Any advice would be appreciated. This seems to be common on these bikes....My last dirt bike was a kdx175 20 years ago...lol.....now Im 40yrs old 260 lbs and have had 2 minor knee surgeries.....What have I got myself into.....lol

Edited July 14, 2014 by nohope

[trailboss14]

Yeah the gear is likely worn out. Same thing happened to my buddies KTM 250sx-f. You're probably going to have to pull the motor and split the case to see the damage. Takes some time if you do it yourself... Anyway, here is a link to Motosport for the parts you may need.

 

http://www.motosport.com/dirtbike/oem-parts/YAMAHA/2005/YZ450F/TRANSMISSION

[nohope]

Yeah the gear is likely worn out. Same thing happened to my buddies KTM 250sx-f. You're probably going to have to pull the motor and split the case to see the damage. Takes some time if you do it yourself... Anyway, here is a link to Motosport for the parts you may need.

 

http://www.motosport.com/dirtbike/oem-parts/YAMAHA/2005/YZ450F/TRANSMISSION

Thanks man....I've got a friend who can help me

[nohope]

Got the motor out this afternoon. Next up tear down time.

[nohope]

Here is an update:

Before we split the cases we found what appeared to be old gasket material or metal between the shift drum and the segment flat area.  We also found some metal shavings to around the segment flat area. Upon seperating the cases we noticed that the rear dowl pin was gone and there was sealent between the cases...no gasket. Next the 3rd and fourth gear cogs look fine but there was some wear on the 1st gear shift fork and a little wear on the 3rd and 4th gear shift fork I have nothing to compare them too so Im not sure if they are bent...So what was causing my bike to jump in and out of 4th gear?

Edited July 23, 2014 by nohope

[grayracer513]

Both the forks are bent.  You can tell by the wear pattern.

 

The wear on the gears that cause this problem (and bent the fork) is not on the teeth, but the lugs and slots on the sides. 

[nohope]

Both the forks are bent.  You can tell by the wear pattern.

 

The wear on the gears that cause this problem (and bent the fork) is not on the teeth, but the lugs and slots on the sides. 

There is absolutely no wear on the gear dogs  and slots.  But I could tell that the cases have been apart recently and it almost looks like the gears were replaced but not the shift forks...?

do you think because the rear dowel pin was missing that the cases could have been misaligned causing some of the problem?....I really don't know what to do next....the forks don't appear to be bent but the wear marks tell me something isn't right

Edited July 23, 2014 by nohope

[grayracer513]

I doubt that the missing dowel contributes anything to the problem, but it can't be a good thing.  Be sure to replace it.

 

If the gears look that good, try it with a couple of new forks and see what happens.  They don't have to be folded over or anything to cause trouble.  If the fingers of the fork were to move as much as a millimeter, it can cause the trans not to hold in gear.  Ideally, only the flattened tips of the fork would show any wear.

[nohope]

Are the new shift forks stronger? I don't want to tear the cases apart every year or so.  I would hope Yamaha has corrected the problem by now. It would be nice to just by better shift forks and be done with it.

[grayracer513]

The shift forks were most likely bent by a previous worn dog condition that cause gears to push out of gear under a load, then the person making the previous repair probably neglected to replace them.  Fork strength isn't an issue unless the trans gets worn like that.

 

What causes the gears to wear like that is unclear, but IMO, it's a matter of how the bike gets shifted over time.  Shifting firmly, positively, and cleanly, so as not to allow the gears to clash against one another while trying to lock up is, IMO, the way to avoid the problem.  "Lazy" shifting and/or missing gears all the time will speed up the wear problem.  Stopping on the lever all the time may end up bending a fork.  On my own bike I had over 300 hours on the original trans without any kind of trouble, and when I changed out 4th and 5th for different ratios, the gearbox basically looked new. 

[nohope]

a piece of metal stuck between the shift segment and the shift fork cam was causing the cam to be out of align....kinda cocked over a little. This in IMO could keep the fork in a bind. Do I use a case gasket or sealer?

[grayracer513]

There is no case gasket.  Use YamaBond, or Permatex "Right Stuff" RTV (thin layer), or their anaerobic joint sealer.

 

The shift cam cannot have been misaligned with the forks by anything placed between the segment and the cam because the cam is fully supported by the two bearing points that carry it in the crankcases.  The forks shafts, both transmission shafts, and the cam are all located on their axes by the same to parts, so if one is out of position, the other 4 are still in parallel.  The only thing that such a thing could have done is to allow the cam to drift toward the left side by whatever amount the segment was lifted away from the cam.  That drift would have been limited by the clearance between the left end of the cam and the case.  It's possible it would have been enough to cause the problem, but the wear on the forks belies that.  They're bent.

[nohope]

OK....Ive screwed up somewhere. Got the motor back together and put it on the frame. Made a couple test runs down my 400ft driveway and it is hard to find neutral and I can shift into 1st then the next 2 shifts is neutral and then it goes into 4th....&%$#@!....we shifted it through the gears with the shift segment on the table and later with the shift lever and it didn't want to shift smooth. So I put it on the bike with some fresh oil thinking it needed some more lubrication but I guess something is wrong. We installed both new forks and a used shift cam that looked new...because I ordered 2 used shift forks and a shift cam but it got lost in the mail and then I ordered 2 new forks and used the used shift cam. The used shift forks showed up a day after I bought the new ones...I used the new ones just in case....Im stumped

Edited August 20, 2014 by nohope

[nohope]

any suggestions? Im hoping I can pull the side cover off and find a problem with the shift segment. It was a pain to install

Edited August 20, 2014 by nohope

[nohope]

ok got the motor back together after pushing the shift drum dowl pin almost flush with the surface while instaliing the shift segment. I replaced the shiftdrum and got motor back together and on the bike and it shifts great... but it still jumps out of 4th but not as frequent.....I installed 2 new shift forks. The 3rd and 4th gear dogs had little if any wear on them.....I mean so little that I couldn't see the any wear on them  Im lost.....I've  spit the cases 2 times this week and Im stumped. The rear drive sprocket has some wear and the chain is adjusted according to the book....I can pull the chain off the sproket about 3/4 to an 1 inch at the 3 oclock position....Ive read that this is too much slack and the chain is stretched and may be my problem....Ive spent way more money than I will ever get out of this bike and I have ran out of ideas.......also there are a bunch of lugs torn off the rear tire but that probably has nothing to do with the 4th gear issue but at this point I will try anything.

Edited August 25, 2014 by nohope